I picked up a Uniden BC125AT and programmed in the BNSF frequencies for Denver, CO. For what it's worth, I live in central Denver, about a mile east of the BNSF Joint Line.

Well, I took the scanner out tonight and biked to the 8th Avenue bridge, which crosses over the tracks. I had my scanner on the whole time. All I heard was occasional static and rare voice traffic , most of it unintelligible.

There was very little going on. Anyway, I'm on the bridge, waiting... I'm up pretty high, over the tracks, near downtown, so I would imagine that I would have a good signal.

After about 10 minutes a loaded coal drag passes under the bridge. There was nothing on the radio that suggested that would be happening.

Up here in Wyoming, radio chatter is mostly switching cars,loading, unloading, or track information. As you are close to the tracks,I would suggest just leaving the scanner on when you are home. An outside antenna would ensure reception but a mile from the tracks you should be fine. As already mentioned, it is not constant chatter and a week or so of scanning should let you know if you want to continue.

If radio traffic is light,I suggest you program all the 'channels' used by railroads as they may be using other channels. If you 'find' them submit the data here so the database can be updated. That's what RR is-people reporting frequencies so others can use the information.

As a conductor we don't always use the radio for things. Usually DS will give us updated info for slow orders, at grade crossing gate malfunctions, weather warnings and such. If you are in an area controlled by CTC, the train crews will operate on signal indication. I can go my entire trip almost 300 miles and never talk on a radio, some days it seems like I am talking and copying all the mentioned above items.

A case-and-point is the UP Overland Route across Wyoming. A constant parade of trains, but outside of Cheyenne Yard and the stop for fuel at Rawlins, there is very little radio chatter. Detectors talk on defect only, and maintainers get track and time over the phone.

Another thing you could do is figure out the closest defect detector to your location on the line your wanting to listen to. That will allow you to see if your monitoring the right frequency and if your a little ways from it or out of site of it but can still pick it up it'll give you a heads up of what's coming. Also stick in 452.9375 & 457.9375 which are the E.O.T. (End Of Train) device frequencies. They are low powered data transmissions from the head end to the F.R.E.D. & vice versa and are a great way to know a train is coming before you see it. However in a yard setting they are useless cause your picking up multiple signals from multiple trains lol!!!!!!!

You might try that feed for a few days to get an idea of what to expect.

Also, if you plan on just using your scanner for railroad monitoring, you may want to consider getting a slightly higher-gain antenna tuned specifically for the railroad band. The ones sold by Smiley Antenna are quite good.

Have you checked out the frequency database?
Here in northern NJ, I monitor mainly CSX, but have all of the local rr's programmed into my radios.
CSX calls out every signal regardless of display, and they have defect detectors that transmit on the road channel.
I use a Yaesu/Vertex VX-150 (2m transceiver) and a Yaesu FT-60R (2m/70cm transceiver) for monitoring railroads. I have an outdoor antenna (a Cushcraft ARX-2B) about 30ft up, and can receive signals as far as 30 miles away, but that depends on several factors, including train's radio/antenna, location/terrain, weather, and time of year (foliage).

In Denver, you should have no trouble at all hearing signals from the east, but from the west it would be very difficult to hear anything beyond the foothills of the Rockies.
As was already said, class 1's are CTC, so, unless they are required to call out signals, they don't require use of the radio unless there is a train order, change of route/track or an emergency.
You should be able to pick up the defect detectors, although most of them transmit very low power, so are limited distance for your receiver.

If you have the Freq's correct, you should be hearing a lot of things. The Joint line uses TWC from Littleton all the way to Pueblo. Not to mention, you should be hearing a bunch of Yard traffic. You mentioned you were listening from a bridge. What was your location? BTW, Neither BSNF or UP are using digital transmissions here in Colorado.

You are getting excellent advice here, especially from those in the business. I program the entire AAR band-plan just in case frequencies change. I also program all the UHF frequencies in a separate bank so when things quiet down considerably on VHF I combine the VHF and UHF banks for EOT or those annoying switch control data signals on VHF, both of which will at least let you know your radio is working. I do this with aviation stuff too. I don't do ATC but only airline company voice frequencies exclusively. ACARS goes into a separate bank and I combine that bank after the Red Eyes arrive and depart and I am not watching FlightRadar24. Take care.